There is conventionally known a serial printer which receives print data from a host computer (to be referred to as a host hereinafter), conveys a printing medium such as a printing sheet, and prints data while reciprocally scanning a printhead by using the received data.
The serial printer repeats data reception from the host and printing by the printhead. To adjust the reception speed and printing speed, the serial printer generally employs a data buffer which stores print data necessary for printing by at least one scanning of the printhead.
As the printing width of the printhead becomes longer or the number of print elements increases for a higher printing resolution, the data buffer capacity also increases. The printer must be equipped with an expensive, large-capacity memory, thus resulting in increasing the printer cost.
To suppress an increase in cost along with an increase in data buffer capacity, the data amount along the array direction (sub-scanning direction) of print elements of the printhead per scanning/printing of the printhead is intentionally decreased in a printer with small data buffer memory capacity.
Some printers employ a data buffer whose capacity is smaller than print data necessary for printing by one scanning of the printhead.
Recently, the printing resolution of printing apparatuses such as a printer which prints digital images is increasing along with the spread of personal computers, digital cameras, and the like. Of printing apparatuses, inkjet printing apparatuses using an inkjet printhead rapidly prevail. Demands further arise for lower-cost inkjet printing apparatuses which realize high-resolution images.
A serial scanning inkjet printing apparatus prints an image by one scanning at the printhead width while scanning the printhead in a direction (main scanning direction) perpendicular to the conveyance direction (sub-scanning direction) of a printing medium such as a printing sheet. At the end of printing by one scanning, the inkjet printing apparatus conveys the printing medium by the printhead width in the conveyance direction. The inkjet printing apparatus sequentially repeats image printing by one scanning and conveyance of the printing medium described above, forming a desired image on the printing medium.
In a conventional printing apparatus, data sent from the host are data sequentially arrayed in the main scanning direction (to be referred to as raster data hereinafter). To print an image by one scanning at the printhead width, data must be converted into data sequentially arrayed in the sub-scanning direction (to be referred to as column data hereinafter) in accordance with the number of print elements (e.g., the number of ink discharge nozzles) of the printhead. To print an image by one scanning, the printing apparatus must hold at least a memory area for accumulating raster data for the number of discharge nozzles and a memory area for accumulating column data.
As described above, higher image quality by higher-resolution printing of the printing apparatus is required. A higher resolution of the printing apparatus increases the data amount necessary for image printing. The memory area necessary for the printing apparatus also increases, which inhibits cost reduction of the printing apparatus.
A technique of reducing the memory area necessary for the printing apparatus is disclosed in Japanese Patent Publication Laid-Open No. 11-259248. According to this publication, the memory capacity necessary for the printing apparatus can be reduced by the following data transfer between the host and the printing apparatus while buffering is minimized.
More specifically, data is transferred between the host and the printing apparatus by dividing image data necessary for printing by one scanning of the printhead into a plurality of blocks (each block will be referred to as a data frame). When the printing apparatus receives image data transferred from the host, the printing apparatus processes the image data and temporarily holds it in the memory area of the printing apparatus. This memory area stores one or more data frames during printing operation by one scanning of the printhead.
In this data transfer in which image data necessary for printing by one scanning of the printhead is divided into a plurality of data frames, data frames of image data are successively transmitted to the memory area of the printing apparatus. The memory area size suffices to ensure at least a memory area capable of storing one or more data frames, and the memory area need not store image data for one scanning. Unlike a conventional printing apparatus, a memory area for storing image data for one scanning is not required.
In the above-described data transfer, printing operation can start upon reception of minimum data necessary for image printing without storing image data necessary for printing by one scanning of the printhead in the memory area of the printing apparatus. Sequentially transmitted data frames are stored in the memory area of the printhead while rewriting the memory area. An image by one scanning can be formed without interrupting scanning of the printhead, while the memory capacity necessary for the printing apparatus can be reduced.
If print data is not completely mapped in the buffer during printing by scanning the printhead in the use of a small-capacity buffer which cannot store print data for one scanning, data is printed up to the mapped position, and scanning/printing stops. After the remaining print data is mapped in the buffer, the printhead must be scanned from the position where previous printing ends, restarting printing. When the printhead is scanned to print data upon reception of print data for less than one scanning from the host apparatus, subsequent print data may be mapped in the buffer area at the end of printing, and printed. This arrangement also suffers the same problem when a communication error occurs between the host and the printing apparatus or when the next print data cannot be completely transferred before the completion of printing by one scanning because of a low transfer speed.
In the above arrangement in which printing is interrupted during scanning and restarts after print data is transferred, a printed image until the stop of printing/scanning and a printed image after the restart of printing/scanning may overlap each other, or a gap may be formed between the two printed images due to the influence of the operation precision of a mechanism of scanning the printhead and the control precision of the moving speed of the printhead in the scanning direction (main scanning direction), thus degrading the image quality.
In addition, in the above-described data transfer, a job and task in the host cannot be managed by the printing apparatus. Data transfer from the host to the printing apparatus may be left undone due to some reasons.
For example, if data transfer from the host to the printing apparatus is left undone after the start of printing operation by one scanning, the printing apparatus cannot print any image by the printhead. At this time, the printhead must stop at the position until the next image data is received, or must return to the home position (reference position) and wait until image data is stored in the memory area. After a sufficient amount of image data is stored in the memory area, the printhead is scanned again. In other words, the printing apparatus restarts printing from the position where printing stops due to the interruption of data transfer from the host, and completes image printing by one scanning.
If, however, image printing is interrupted during one scanning, a difference (printing time difference) in the printing time on a printing medium is generated between a dot printed at the interrupted position and a dot printed at a printing restart position adjacent to the interrupted position. It is generally known that the density change caused by the printing time difference appears as density unevenness (time difference unevenness) in a printed image due to the printing time difference.
The reason is as follows.
Ink droplets attached to a printing medium permeate in a direction perpendicular to the printing medium (e.g., paper) (direction of thickness of the printing medium) and a direction in which ink droplets spread on the surface of the printing medium. A pigment such as a dye serving as an ink component is physically and chemically bonded with the printing medium.
If a time difference (t2−t1) between printing times (t1 and t2) when two adjacent dots are printed is small, ink droplets attached to the printing medium later (t2) also permeate in the direction perpendicular to the printing medium and the direction in which ink droplets spread on the surface of the printing medium. However, these ink droplets hardly permeate and fix in a region where ink droplets attached earlier (t1) fix.
This is because ink droplets attached earlier (t1) are still permeating, and a limited amount of ink component can be chemically bonded with the printing medium. Thus, ink droplets attached later (t2) permeate and fix further below the region where ink droplets attached earlier (t1) permeated.
If the time difference (t1−t2) between the printing times (t1 and t2) when two adjacent dots are printed is large, ink droplets attached later (t2) permeate by a larger amount than ink droplets for the small time difference (t2−t1) in the region where ink droplets attached earlier (t1) permeated and fixed.
This is because ink droplets attached earlier (t1) sufficiently permeated and spread, or the volatile component of ink droplets evaporated, the ink droplet amount per unit area decreases, and ink droplets attached later (t2) can spread and penetrate to the region of ink droplets attached earlier (t1).
More specifically, if the printing time difference between adjacent dots is large, a large amount of ink, i.e., a pigment or ink component such as a dye fixed near the surface of the printing medium remains. The density depends on light absorption of the pigment fixed near the surface of the printing medium, and a larger printing time difference leads to a higher density. If printing operation is interrupted during one scanning, density unevenness occurs in images printed before and after interruption, and the boundary between the images before and after interruption appears as a stripe. This greatly degrades the quality of the printed image.
[Image Quality When Data Transfer is Left Undone (FIGS. 18A and 18B)]
Degradation of the image quality when data transfer from the host to the printing medium is left undone, as described above, will be explained in detail with reference to FIGS. 18A and 18B.
In FIG. 18A, image data of one scanning is divided into n data frames. Data frames of image data of one scanning transmitted from the host to the printing apparatus are transmitted in an order of 301, 302, . . . , 30n in FIG. 18A. According to FIG. 18A, the printing apparatus starts printing an image for one scanning at the printhead width upon reception of data up to, e.g., the data frame 303.
At this time, an image is printed sequentially from the data frame 301 on a printing medium, as shown in FIG. 18B. Image data accumulated in the memory area is decreased with the progress of image printing. Data frames sequentially transmitted from the host are stored in the memory area while the contents of the memory area of the printing apparatus are sequentially rewritten.
A case where data transfer from the host is left undone when the printing apparatus receives the data frames up to the data frame 305 shown in FIG. 18A will be explained.
If data transfer from the host is left undone when the printing apparatus receives the data frames up to the data frame 305, the printing apparatus interrupts image printing operation at the end of image printing using the data frame 305, halts the movement of the printhead at the current position or returns the printhead to the reference position, and waits until data is transferred again from the host. After data transfer from the host restarts and image data of the data frame 306 and subsequent data frames are accumulated by a necessary amount in the memory area of the printing apparatus, the printing apparatus restarts image printing from the interrupted position.
As shown in FIG. 18B, density unevenness occurs between dots (attached ink droplets) printed near the boundary between images 305 and 306 printed using the data frames 305 and 306 which have a printing time difference due to the interruption of data transfer from the host. This unevenness is recognized as a stripe near the boundary between the images 305 and 306. The stripe greatly degrades the image quality.